Phase-shifted monomode optical fibre with large effective area

ABSTRACT

A monomode dispersion shifted optical fibre having an effective core area greater than 100 μm 2 , characterized in that it has a chromatic dispersion cancellation wavelength λ 0  from 1 400 nm to 1 500 nm and low curvature losses.

BACKGROUND OF THE INVENTION

The present invention relates to optical fibres and in particular todispersion shifted fibres intended to be used in wavelength divisionmultiplex transmission systems.

Monomode optical fibres referred to as dispersion shifted fibres (DSF)are used at wavelengths around 1 550 nm, and more generally in a windowof wavelengths from 1 500 nm to 1 600 nm. Silica has non-zero chromaticdispersion at these wavelengths (in contrast to its chromatic dispersionin the transmission wavelength window around 1 300nm). The chromaticdispersion of the silica is compensated, in particular by increasing theindex difference Δn between the core of the fibre and its opticalcladding, to limit chromatic dispersion of the wave transmitted in thetransmission window of dispersion shifted fibres. In practice this indexdifference shifts the wavelength at which chromatic dispersion iscancelled out; it is obtained by introducing dopants into the fibreduring its fabrication, for example by an MCVD process known in the art,which is not described in more detail here. A typical value for theindex difference between the cladding and the core of the fibre is10×10⁻³ to 14×10⁻³ ; the index can be increased in the silica by usinggermanium as the dopant.

Monomode dispersion shifted fibres must also have low curvature lossesand low attenuation, just like conventional line fibres.

What is more, using dispersion shifted fibres in wavelength divisionmultiplex transmission systems which transmit RZ, NRZ or soliton pulsesis subject to further constraints, all the more so as the number ofchannels transmitted, the bit rate of each channel and the power on theoutput side of the amplifier increase and the spacing between channelsdecreases. Thus it is preferable to use a fibre having sufficiently highchromatic dispersion in the transmission window to prevent thephenomenon of four-wave mixing. Fibres are therefore used which have achromatic dispersion cancellation wavelength λ₀ other than 1 550 nm toprevent the problems caused by four-wave mixing. These fibres arereferred to as non-zero dispersion shifted fibres (NZ-DSF).

Finally, to avoid non-linear effects, the fibres must have a largeeffective core area, typically greater than 70 μm².

M. Kato et al., “A new design for dispersion shifted fibre with aneffective core area larger than 100 μm² and good bendingcharacteristics”, ThK1, OFC'98 Technical Digest, explains thatnon-linear effects in the fibres could become the dominant limitation ontransmission capacity and distance for long-haul high-capacity amplifiedtransmission systems. The document specifies that one possible solutionis to increase the effective core area of the fibres, which produces ahigher power and a greater distance between repeaters. The documentproposes a fibre having a profile referred to as a coaxial profile,surrounded by a pedestal, with an effective core area of 146 μm² and achromatic dispersion cancellation wavelength λ₀ of 1 500 nm. Thechromatic dispersion at 1 550 nm is low and the dispersion slope at thiswavelength is 0.09 ps/nm².km.

EP-A-0 789 255 describes dispersion shifted fibres with high effectivecore areas, greater than 200 μm². The fibres have wavelengths λ₀,greater than 1 550 nm. One example of a family of fibres has a value ofλ₀ at 1 580 nm, an effective core area of 265 μm² and a chromaticdispersion slope of 0.085 ps/nm².km.

The fibres described in the above two prior art documents have thedisadvantage of very low chromatic dispersion at 1 550 nm; this value ofthe chromatic dispersion at 1 550 nm does not prevent four-wave mixing.

SUMMARY OF THE INVENTION

The object of the invention is to provide a dispersion shifted fibrewith a high effective core area which also has sufficiently highchromatic dispersion at 1 550 nm to prevent four-wave mixing.

To this end the invention proposes a monomode dispersion shifted opticalfibre having an effective core area greater than 100 μm², characterizedin that it has a chromatic dispersion cancellation wavelength λ₀ from 1400 nm to 1 500 nm and low curvature losses.

In one embodiment of the invention, the chromatic dispersioncancellation wavelength λ₀ is from 1 450 nm to 1 500 nm and preferablyapproximately 1 480 nm.

The fibre advantageously has a chromatic dispersion from 7.5 ps/nm.km to10 ps/nm.km for a wavelength of 1 550 nm.

In one embodiment of the invention, the fibre has an effective core areafrom 110 μm² to 125 μm².

In another embodiment of the invention, 100 turns of fibre with a radiusof 30 mm have an attenuation less than or equal to 0.1 dB at awavelength of 1 670 nm.

A first embodiment of a fibre according to the invention has an indexprofile with a core surrounded by an optical cladding, said coreincluding, coaxially and starting from the axis of the fibre:

a trapezium-shaped central part,

an intermediate area in which the index is lower than the maximum indexof the central part, and

a ring in which the index is lower than the maximum index of the centralpart and higher than that of the intermediate area.

The profile advantageously includes between the ring and the cladding asecond intermediate area in which the index is lower than that of thecladding.

In a second embodiment of the invention, the fibre has an index profilewith a core surrounded by an optical cladding, said core including,coaxially and starting from the axis of the fibre:

a central part in which the index is lower than or substantially equalto that of the cladding,

a peripheral area in which the index is higher than that of the centralpart,

an intermediate area in which the index is lower than that of thecentral part, and

a ring in which the index is higher than that of the cladding.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent onreading the following description of embodiments of the invention, whichdescription is given by way of example and with reference to theaccompanying drawings, in which:

FIG. 1 is a diagrammatic representation of the index profile of a firstembodiment of a fibre according to the invention, and

FIG. 2 is a diagrammatic representation of the index profile of a secondembodiment of a fibre according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The fibre according to the invention has a large effective core area,typically greater than 100 μm², and a chromatic dispersion cancellationwavelength λ₀ from 1 400 nm to 1 500 nm. A value of λ₀ from 1 450 nm to1 500 nm, for example around 1 480 nm, as in the example shown in thefigure, proves to be appropriate for wavelength division multiplextransmission. Effective core areas from 110 μm² to 125 μm² are suitable.

The invention further proposes that the chromatic dispersion at 1 550 nmbe from 7.5 ps/nm.km to 10 ps/nm.km. Those values limit four-wave mixingin the current transmission wavelength ranges and in particular from 1530 nm to 35 1 580 nm for wavelength division multiplex transmissionsystems.

The invention finally proposes a fibre whose sensitivity to curvaturesis similar to that of prior art fibres, or even better. Values of λ_(S)beyond 1 600 nm, and for example of the order of 1 670 nm, areappropriate. The wavelength λ_(S) is the wavelength at which theattenuation caused by winding 100 turns of fibre with a radius of 30 mmreaches 0.1 dB. The invention therefore provides a fibre having anattenuation increment of less than 0.05 dB at 1 550 nm for 100 turns offibre with a radius of 30 mm.

FIG. 1 is a diagrammatic representation of an index profile 10 that canproduce the features of the invention. The radii in μm are plotted onthe abscissa axis and the index, defined by its difference relative tothe index of the cladding of the fibre, either as a relative value (Δn)or as a percentage (Δn%=100. Δn/n), is plotted on the ordinate axis.

The index profile shown in FIG. 1 is a trapezium+ring index profile. Itincludes, starting from the centre of the fibre and in the direction ofthe cladding, a central part 11 ₁ having a substantially constant index,or to be more precise a substantially constant index difference Δn,extending out to a radius r₀. The index Δn₁ is higher than the indexn_(c) of the silica of the cladding 15. In the embodiment shown in thefigure, the index Δn₁, is from 0.75% to 0.85% and the radius r₀ is 0.84μm.

Around this central part 11 ₁ in which the index is higher than that ofthe cladding 15, the fibre of the invention has a part 11 ₂ whose indexdecreases linearly or substantially linearly between the radii r₀ andr₁; this linearly decreasing index part 11 ₂ and the central part 11 ₁′together constitute a trapezium 11. The index r₁ can take values from2.75 μm to 2.85 μm; the constant index part of the trapezium 11, i.e.the top of the trapezium, extends approximately 0.3 times the totalradius of the trapezium, representing a ratio r₀ /r₁ from 0.2 to 0.5,for example.

The fibre then has a first intermediate area 12 between radii r₁ and r₂in which the index Δn₃ is substantially constant and lower than or equalto the index of the cladding 15. In the embodiment shown in the figure,the index difference Δn₃ is −0.02% and the radius r₂ can take valuesfrom 4.8 μm to 5.1 μm. More generally, values of Δn₃ between −0.01% and−0.03% are appropriate.

The fibre has around the first intermediate area 12 a ring 13 in whichthe index Δn₂ is higher than that of the cladding 15. In the FIG. 1embodiment, the ring 13 has an index Δn₂ from 0.45% to 0.52% betweenradii r₂ and r₃. The index Δn₂ of the ring is preferably less than orequal to the index Δn₁of the central part 11 ₁ of the trapezium. Theoutside radius r₃ of the ring 13 is preferably from 6.1 μm to 6.6 μm.

Outside the ring 13, before the cladding 15, the fibre has a secondintermediate area 14 in which the index is lower than or equal to thatof the cladding 15. In the FIG. 1 example, the second intermediate area14 has the same index Δn₄ of −0.02% (or preferably from −0.03% to−0.01%) as the first intermediate area 12 between the trapezium 11 andthe ring 13. The second intermediate area 14 lies between the radii r₃and r₄ where the value of r₄ is from 12.4 μm to 13.1 μm.

The chosen index profile gives the fibre a high effective core area,from 110 μm² to 125 μm², and a chromatic dispersion cancellationwavelength λ₀ from 1 450 nm to 1 480 nm. It is therefore possible toincrease the power of the amplifiers in a wavelength division multiplextransmission system and to reduce the distance between repeaters of atransmission system using the optical fibre of the invention as thetransmission medium.

In the foregoing example, the chromatic dispersion at 1 550 nm is 8.8ps/nm.km and is thus between the preferred values from 7.5 ps/nm.km to10.0 ps/nm.km.

The attenuation in the fibre is advantageously low enough to providegood in-cable performance; to this end it is advantageous for thewavelength λ_(S) of the fibre to be greater than or equal to 1 670 nmor, which amounts to the same thing, for the attenuation due tocurvatures to be less than 0.1 dB below that wavelength.

Another example of a trapezium+ring profile like that shown in FIG. 1has the parameters set out in table 1 below.

TABLE 1 r₀ (μm) r₁ (μm) r₂ (μm) r₃ (μm) 10³ × Δn₁ 10³ × Δn₂ 10³ × Δn₃10³ × Δn₄ 0.68 2.27 4.61 5.98 13.7 8.5 0 0

The propagation characteristics of a fibre having the table 1 parametersare set out in table 2 below, in which the following notation is used:

λ_(C): theoretical cut-off wavelength,

λ₀: chromatic dispersion cancellation wavelength,

CD/dλ: chromatic dispersion slope at 1 550 nm,

CD: chromatic dispersion at 1 550 nm,

W₀₂: mode diameter at 1 550 nm,

S_(eff): effective core area at 1 550 nm,

S_(C): sensitivity to curvatures at 1 550 nm for 100 turns with a radiusof 30 mm, and

S_(μc): sensitivity to microcurvatures at 1 550 nm as a proportionrelative to the G652 fibre sold by the applicant.

TABLE 2 CD/dλ CD 2W₀₂ S_(eff) S_(c) λ_(c) (nm) λ₀ (nm) (ps/nm² · km)(ps/nm² · km) (μm) (μm²) (dB) S_(μc) 1 780 1 475 0.1 8 10.6 109 <10⁻³1.2

The theoretical cut-off wavelength is generally two to four hundrednanometres greater than the cut-off wavelength as measured in the fibrewhen incorporated into the cable containing it. Accordingly, thein-cable cut-off wavelength of the fibre according to the invention isless than 1 500 nm and the fibre according to the invention iseffectively a monomode fibre throughout the range of wavelengths of themultiplex.

More generally, the parameters of fibres according to the inventionwhich have a trapezium+ring profile can be chosen as follows to satisfythe requirements of the invention:

9×10⁻³ ≦Δn ₁≦17×10⁻³

3×10⁻³ ≦Δn ₂≦10.5×10⁻³

−0.5×10⁻³ ≦Δn ₃≦0.5×10⁻³

Choosing index differences from the above ranges yields the followingranges for the radii:

0.35≦r ₁ /r ₃≦0.5

0.55≦r ₂ /r _(3≦)0.85

5 μm≦r ₃≦7.5 μm

FIG. 2 is a diagrammatic representation of an index profile 20 that canproduce the features of the invention. The radii in μm are plotted onthe abscissa axis and the index as an absolute difference Δn relative tothe index of the optical cladding is plotted on the ordinate axis.

The FIG. 2 index profile is a coaxial index profile. The profile 20includes, starting from the centre of the fibre and in the direction ofthe cladding, a central part 21 having a substantially constant index,or to be more precise a substantially constant index difference Δn₁,extending out to a radius r₁. The index Δn₁, is lower than orsubstantially equal to the index n_(C), of the silica of the cladding25. The fibre then has a peripheral area 22 between radii r₁ and r₂ inwhich the index Δn₂ is substantially constant and higher than or equalto the index of the cladding 25. Around the peripheral area 22 is anintermediate area 23 between radii r₂ and r₃ in which the index Δn₃ islower than or equal to that of the cladding 25. Finally, between theintermediate area 23 and the cladding 25, the fibre includes a ring 24between radii r₃ and r₄ in which the index is Δn₄. The parameterscharacteristic of two examples of fibres according to the inventioncorresponding to the FIG. 2 profile are set out in table 3 and therespective propagation characteristics of the fibres are set out in thecorresponding rows of table 4.

TABLE 3 r₁ (μm) r₂ (μm) r₃ (μm) r₄ (μm) 10³ × Δn₁ 10³ × Δn₂ 10³ × Δn₃10³ × Δn₄ 3.99 5.71 13.86 16.3 0 11.35 −34 2.25 2.13 3.55 5.68 7.1 011.7 −2.35 5.85

TABLE 4 CD/dλ CD 2W₀₂ S_(eff) S_(c) λ_(c) (nm) λ₀ (nm) (ps/nm² · km)(ps/nm² · km) (μm) (μm²) (dB) S_(μc) 1 710 1 460 0.084 8 8.7 137 <10⁻⁵1.3 1 705 1 455 0.081 8 10 102 <10⁻⁵ 1

More generally, the parameters of fibres according to the inventionhaving a coaxial+ring profile can be chosen as follows to satisfy therequirements of the invention:

−7×10⁻³ ≦Δn _(1≦)0.5×10⁻³

10×10⁻³ ≦Δn ₂≦17×10 ⁻³

−7×10⁻³ ≦Δn ₃≦−2×10⁻³

1×10⁻³ ≦Δn ₄≦6×10⁻³

Choosing index differences in the above ranges yields the followingranges for the radii:

0.4≦r ₁ /r ₂≦0.7

0.3≦r ₂ /r ₄≦0.6

0.6≦r ₃ /r ₄≦0.9

6 μm≦r ₄≦17 μm

Fibres with profiles of the FIG. 2 type have the same advantages asthose with profiles of the FIG. 1 type.

The skilled person can put the invention into practice using processesknown in the art, such as the MCVD process or other processes routinelyused to fabricate optical fibres.

Of course, the present invention is not limited to the examples andembodiments described and shown, and is open to many variants that willsuggest themselves to the skilled person. Thus the invention has beendescribed in its preferred application to compensating dispersion inwavelength division multiplex transmission systems. It has otherapplications.

The FIGS. 1 and 2 profiles constitute examples enabling implementationof the invention. Other profiles can produce the slope and dispersionvalues proposed in accordance with the invention.

What is claimed is:
 1. A monomode dispersion shifted optical fibrehaving an effective core area greater than 100 μm², characterized inthat said fiber has a chromatic dispersion cancellation wavelength λ₀from 1400 nm to 1500 nm and low curvature losses.
 2. A fibre accordingto claim 1, characterized in that it has an index profile with a coresurrounded by an optical cladding, said core including, coaxially andstarting from the axis of the fibre: a trapezium-shaped central part, anintermediate area in which the index is lower than the maximum, index ofthe central part, and a ring in which the index is lower than themaximum index of the central part and higher than that of theintermediate area.
 3. A fibre according to claim 2, characterized inthat the profile includes between the ring and the cladding a secondintermediate area in which the index is lower than or equal to that ofthe cladding.
 4. A fibre according to claim 2, characterized in that:the index difference between said central part and said cladding is from9×10⁻³ to 17×10⁻³, the index difference between said ring and saidcladding is from 3×10⁻³ to 10.5×10⁻³, and the index difference betweensaid intermediate area and said cladding is from −0.5×10⁻³ to 0.5×10⁻³.5. A fibre according to claim 1, characterized in that it has an indexprofile with a core surrounded by an optical cladding, said coreincluding, coaxially and starting from the axis of the fibre: a centralpart in which the index is lower than or equal to that of the cladding,a peripheral area in which the index is higher than that of the centralpart, an intermediate area in which the index is lower than that of thecentral part, and a ring in which the index is higher than that of thecladding.
 6. A fibre according to claim 5, characterized in that: theindex difference between the central part and the cladding is from−7×10⁻³ to 0.5×10⁻³, the index difference between said peripheral areaand the cladding is from 10×10⁻³ to 17×10⁻³, the index differencebetween said intermediate area and the cladding is from −7×10⁻³ to−2×10⁻³, and the index difference between said ring and the cladding isfrom 1×10⁻³ to 6×10⁻³.
 7. A fibre according to claim 1, characterized inthat the chromatic dispersion cancellation wavelength λ₀ is from 1450 nmto less than 1500 nm.
 8. A fibre according to claim 1, characterized inthat it has a chromatic dispersion from 7.5 ps/nm.km to 10 ps/nm.km fora wavelength of 1 550 nm.
 9. A fibre according to claim 1, characterizedin that it has an effective core area from 110 μm² to 125 μm².
 10. Afibre according to claim 1, characterized in that 100 turns of fibrewith a radius of 30 mm have an attenuation less than or equal to 0.1 dBat a wavelength of 1 670 nm.
 11. The fibre according to claim 1, whereinthe chromatic dispersion cancellation wavelength λ₀ is approximately1480 nm.
 12. The fibre according to claim 1, wherein the chromaticdispersion cancellation wavelength λ₀ is approximately from 1450 nm to1480 nm.
 13. A monomode dispersion shifted optical fibre having aneffective core area greater than 100 μm², characterized in that saidfibre has a chromatic dispersion cancellation wavelength λ₀ from 1400 nmto 1500 nm and low curvature losses; and wherein said fibre has an indexprofile with a core surrounded by an optical cladding, said coreincluding, coaxially and starting from the axis of the fibre: atrapezium-shaped central part, an intermediate area in which the indexis lower than a maximum index of the central part, and a ring in whichthe index is lower than the maximum index of the central part and higherthan that of the intermediate area.
 14. The fibre according to claim 13,characterized in that the profile includes between the ring and thecladding a second intermediate area in which the index is lower than orequal to that of the cladding.
 15. The fibre according to claim 13,characterized in that: the index difference between said central partand said cladding is from 9×10⁻³ to 17×10⁻³, the index differencebetween said ring and said cladding is from 3×10⁻³ to 10.5×10⁻³, and theindex difference between said intermediate area and said cladding isfrom −0.5×10⁻³ to 0.5×10⁻³.
 16. A monomode dispersion shifted opticalfibre having an effective core area greater than 100 μm², characterizedin that said fibre has a chromatic dispersion cancellation wavelength λ₀from 1400 nm to 1500 nm and low curvature losses; and wherein said fibrehas an index profile with a core surrounded by an optical cladding, saidcore including, coaxially and starting from the axis of the fibre: acentral part in which the index is lower than or equal to that of thecladding, a peripheral area in which the index is higher than that ofthe central part, an intermediate area in which the index is lower thanthat of the central part, and a ring in which the index is higher thanthat of the cladding.
 17. The fibre according to claim 16, characterizedin that: the index difference between the central part and the claddingis from −7×10⁻³ to 0.5×10⁻³, the index difference between the peripheralarea and the cladding is from 10×10⁻³ to 17×10⁻³, the index differencebetween the intermediate area and the cladding is from −7×10⁻³ to−2×10⁻³, and the index difference between the ring and the cladding isfrom 1×10⁻³ to 6×10⁻³.